High volume conveyor transport for clean environments

ABSTRACT

A segmented, belt-driven conveyor system for use in clean environments. High speed, high density, collision free throughput of work piece carriers is enabled through belt-driven conveyor segments each having co-rotating drive wheels. The drive wheels have a cylindrical profile. Predefined acceleration/deceleration profiles may be employed by a motor controller to affect optimal changes in work piece carrier speed across the respective drive segment. A peripheral groove is formed in idler wheels within a drive segment. A soft, pliant ring of material is disposed in the groove. The ring protrudes slightly beyond the crown of each wheel. The drive belt then remains in contact with the ring when unloaded and the wheel peripheral surface itself when loaded through compression of the pliant ring. By reducing intermittent contact between the belt and the wheels, particulation is reduced.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

BACKGROUND OF THE INVENTION

Many manufacturing factory environments consist of spatially distributedprocessing tools, as opposed to sequential tools located along alinearly arranged assembly line. This is especially true formanufacturing environments where work in process, or a “work entity,”re-enters a tool after being processed by another tool or tools.Re-entry into the same tool avoids tool duplication, which isparticularly important in environments where the capital cost of thetools is high.

A semiconductor manufacturing environment is an example of anenvironment where, due to high tool cost, a work entity enters a giventool, or type of tool, multiple times. Processing tools in asemiconductor manufacturing environment are typically spatiallydistributed in the factory according to function. Thus, the work flowresembles a chaotic movement of the work entity. With multiple workentities being operated upon and moving between multiple tools at thesame time, the respective work flows intersect.

In modern factories, the progress of multiple work entities through thehigh number of manufacturing steps and associated tools is enabled bytransport networks. Simultaneous processing of plural work entities,necessary to maximize usage of the factory tools and to maximize productoutput, results in highly complicated logistics. High efficiency andcoordination in work entity movement is thus required. Without anefficient transportation network capable of rapid, real time response,bottlenecks in the work flow into or out of some process tools candevelop, while other process tools are starved of work. Such anefficient transportation network thus must have high delivery capacity,high speed, and asynchronous capability by which work carriers can moveindependently of each other. The transport infrastructure is theenabling technology for such efficient logistics.

In a recursive process flow environment, such as within a semiconductormanufacturing environment, the simultaneous utilization of up tohundreds of individual process tools requires a logistics network thatis capable of delivering the right work entity at the right time to eachone of the tools. The higher the utilization of each processing tool,the higher the factory output, which simultaneously translates to theincreased efficiency of business capital.

Conveyor systems are one particular type of transportation system usedin contemporary factory environments. A conveyor network may be sharedby several hundred moving work carriers concurrently dispatched tovarious tools. Delivery capacity will depend on flow density andconveyor speed. However, flow density and speed are limited by theadditional requirement of zero tolerance for collisions between workentities within the conveyor system. Thus, a conflict arises between theabove requirements.

A conveyor network typically has intersections, nodes, and branches tomultiple locations in a factory. The open conveyor ends, at workprocessing locations, are the input and output ports for the conveyortransport domain. At these ports, work entities enter and leave theconveyor domain. When a work entity needs to travel from one of theseports to another in the prior art, a path needs to be cleared for thetransit to satisfy the requirement of collision avoidance. Normally,external or centralized dispatch software arranges for such a transit bysimultaneously controlling the movement of all other work entities thatwould otherwise interfere with the work entity in question. Thisdispatch software is complex, due to the aforementioned throughputrequirements. The work entities need be moved concurrently with eachother and at maximum rate without collisions.

In addition to the challenge of highly complex control in densemanufacturing environments, particulate generation by conveyor systemsis of great concern in clean room environments. Thus, the efficiency oftransport systems in such environments must be weighed against theopportunities for contamination.

Traditional roller conveyors have achieved extremely low particulategeneration. However, such arrangements have not been able to achievehigh acceleration of items or carriers transported thereon (genericallyreferred to simply as “carriers” herein) from a stopped condition. Thisis not due to a lack of torque available for the drive rollers butinstead due to the fact that when high starting torque is applied theroller wheels may slip and squeal. This is akin to auto tires squealingwhen accelerating too rapidly from a stop.

In certain embodiments, a hysteresis clutch has been utilized inconjunction with synchronous or stepper motor driven rollers or wheels,depending upon the embodiment, to eliminate such slippage. Hysteresisclutches enable asynchronous soft buffering, a process for movingcarriers independent of each other and starting and stopping thecarriers in a smooth fashion. However, while successful at preventingslippage, hysteresis clutches may make it difficult to achieve highrates of acceleration, including in the multiple g range. Very fastacceleration and deceleration are required in order to increasethroughput and thus the density of carriers traveling on the softbuffered conveyor where carriers must never collide. Since the carriersmove asynchronously, they need to stop fast and short of a collisionwith a downstream carrier to achieve increased density in a conveyorenvironment, as well as start fast so as to minimize interference withupstream carriers.

Principles of physics dictate that the frictional force required to movean object on a surface is dependent on the normal force and thecoefficient of friction for the materials. In other words, it isindependent of the area in contact. However, with compressive materials,higher friction forces can be achieved by selectively increasing thesurface contact. A result of this realization was the increasedutilization of belts for carrier transport, instead of wheels with arubber drive surface in contact with carriers. This increase in surfacearea contact in effect increased the coefficient of friction betweendriving and driven surfaces.

Unfortunately, simply disposing a driving belt on a respective set ofwheels is not clean in terms of particulate generation, particularlywith respect to that resulting from the use of driven and idler wheelsalone. The particulation of the belts results primarily from interactionof the belt with the wheels below the belt, i.e. those supporting theweight of a carrier. Previous investigations into the source ofparticulate generation determined that in many cases the belt was not incontinuous, full contact with the wheels below it due to machiningtolerances in the wheels, the respective axles, and/or the rails thatsupport the wheels. For example, some supporting idler wheels were foundto be in constant contact with the overlying belt and thus were turningin concert with the belt while others started and stopped depending onwhen the belt touched them. The latter contact was haphazard, resultingin frictionally-induced spin up and stops of the supporting idlerwheels. This effect was sometimes dependent upon whether a carrier wasabove the respective portion of belted conveyor.

In order to impart continuous contact between the belt and all of thewheels in a respective conveyor section, including the idler wheels, itwas proposed that the belt be woven in a serpentine path between wheels,such as over two idler wheels and then down under the next. Whilesuccessful in maintaining contact between the belt and all of therespective wheels, this resulted in an increased motor torquerequirement, which also required increased electrical current and thusoperational cost.

There remains the need for an optimized transport solution that resultsin high density, rapid, flexible, and asynchronous work entitytransport, high delivery capacity, avoidance of work entity collisions,and low particulation, particularly for use in clean room environments.

BRIEF SUMMARY OF THE INVENTION

To resolve the inherent conflict between the need for high speed workpiece conveyance and the avoidance of work piece collisions and toincrease throughput, the conveyor infrastructure as presently disclosedis divided into segments, each having a length substantially equivalentto that of a work entity or work piece carrier. A work piece carrier isprevented from entering a conveyor segment if that segment is alreadyoccupied by another work piece carrier. Such collision avoidance isautonomous, embedded in the conveyor itself, allowing a natural,independent flow of dispatched work piece carriers, an approach that isdistinct from the centralized control model as practiced in the priorart. By dividing the longer conveyor runs of the prior art into discretesegments and by enabling intelligent, local control of work piececarriers transiting between segments, the capacity-limiting procedure ofreserving whole conveyor line runs for dispatched work carriers isavoided.

Work piece carriers can be sent from port to port autonomously with highflow densities. With the use of localized, segment-based sensing andconveyor control, carriers can occupy adjacent segments, if needed, andcan pass through nodes on a first come, first served or “natural” basis.

How close work piece carriers can be, on consecutive conveyor segments,a concept referred to as “stacking,” depends in part upon work piececarrier travel velocities, i.e. conveyor speed. In the prior art, theprohibition against entry of a work piece carrier into a zone alreadyoccupied by another work piece carrier demanded generous spacing of thetraveling carriers to ensure sufficient stopping distances to avoidcollisions. The higher the speed, the greater the stopping distance,resulting in less flow density. The limitations on stopping (orstarting) distance in the prior art is a consequence of using rollers todrive the work piece carriers on a conveyor. Yet such rollers werepreviously thought to be the only means of achieving clean, particulatefree movement. In the pursuit of clean transport, roller conveyorsutilized moderate transport speeds to avoid the slipping of the workpiece carriers on the rollers when sudden stopping was necessary toavoid collisions with a downstream, stationary work piece carrier. Thus,the physics of the limited contact surface between work piece carriersand the driving conveyor rollers required such moderate speeds.

With elastic surface contacts, frictional force increases withincreasing surface contact. Thus, to increase driving surface contactbetween the conveyor drive and the work piece carrier, the wheels orrollers of a conveyor in presently disclosed systems and methods aresupplemented with belts of high friction coefficients. However, whileimproving the frictional engagement between work piece carriers and thesegmented conveyor, the introduction of belts may introduce newparticulate sources, particularly with respect to idler wheels, asdiscussed above. Overcoming these difficulties, through developmentsdescribed herein, allows the introduction of high speed, belted, locallycontrolled segmented conveyors providing high rates of work piececarrier acceleration and deceleration in clean manufacturingenvironments. High flow density, at high speeds, thus result.

When velocities are high and stopping and starting distances must beshort, the rate of acceleration and deceleration of the work piececarrier must be limited to avoid slippage on the belt, a condition thatcould create contaminating particulates. Previous control ofparticulation through limited rates of acceleration and decelerationwere achieved through the use of a magnetic hysteresis clutch inconjunction with conveyor segment drive wheels or rollers. The clutchacts as a limiting device on drive roller torque, and can be set todisengage when sudden starting acceleration or rapid stopping of a highspeed motor would otherwise cause the frictional force between theconveyor and the work piece carrier to be exceeded. The application ofsuch a clutch allowed masses and velocities of the work piece carrier tobe variable (e.g., the weight difference between a full work piececarrier versus an empty work piece carrier) while not exceeding amaximum value of inertia.

However, it has been discovered that the use of elastic surface contactbetween a conveyor-driven belt and a work piece carrier providesimproved frictional engagement, thus obviating the need for clutch-basedtechniques for limiting frictional forces. Higher rates of accelerationand deceleration, programmed into local segment controllers, can beemployed, thus improving throughput while avoiding collisions. Suchmotor control can be achieved through servo action or by predefining andlimiting open loop stepper motor rates of acceleration or deceleration.Thus, in a particulate-free clean manufacturing environment, segmentedconveyors with belts, driven by open loop stepper motors or servo motorswith controlled high rates of acceleration and deceleration, results ina collision-free flow of work carriers at high density and high speed,resulting in increased conveyor throughput.

In one particular embodiment, to achieve improved contact between adrive belt and wheels within a respective conveyor section, and idlerswheels in particular, a peripheral groove is formed in each wheeldisposed beneath the belt. A soft, pliant ring of material is thendisposed in the groove. The ring protrudes slightly beyond the crown ofthe idler wheel.

The slight protrusion of the pliant ring results in improved contactwith the drive belt as it passes above the respective idler wheel. Theidler wheels turn in coordination with the drive belt at all times.Particulation is thus significantly reduced and drive motor torquerequirements are also reduced in comparison to the serpentine beltembodiment previously described.

Each pliant ring is configured to achieve constant contact with theoverlying belt when unloaded by a carrier. When a carrier or other itembeing transported is adjacent or above a respective wheel, the pliantring is compressed and the belt comes into contact with the relativelyhard wheel crown or periphery itself, increasing the area of contactbetween the belt and wheel. Thus, the pliant ring material and extent ofprotrusion above the wheel crown are selected to achieve a high degreeof belt contact between the pliant ring and the belt when unloaded anddirect contact between the wheel crown and the belt when loaded. Rapidacceleration and deceleration of carriers is achieved with a relativelylow degree of required torque and with minimized particulation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a section view of a wheel according to the present inventiondisposed from a supporting rail frame;

FIG. 2 is a detailed view of the wheel of FIG. 1;

FIG. 3 is a section view of the wheel of FIG. 1 further illustrating thelocation of a drive belt atop a pliant ring in the wheel;

FIG. 4 is a detailed view of the wheel of FIG. 3;

FIG. 5 is a perspective view of a conveyor drive segment in which isillustrated a drive belt, at least one drive wheel, and a plurality ofidler wheels according to the present invention;

FIG. 6 is a detailed view of the wheel of FIG. 3 under loadedconditions;

FIG. 7 is an plan view of one end of a drive shaft shown in FIG. 5having planar protrusions on opposite ends; and

FIG. 8 is a side perspective view of a drive wheel according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This application claims benefit over U.S. Provisional Application No.61/894,079, filed Oct. 22, 2013, entitled: CONVEYOR SYSTEM PROVIDINGREDUCED PARTICULATION.

FIG. 1 illustrates an idler wheel hub 10 disposed in relation to asupporting rail frame 20. The wheel hub, also simply referred to hereinas “the wheel,” may be formed of a hard, resilient material that isresistant to particulation, such as polyurethane. One preferredembodiment of the wheel 10 employs 75 Shore D cast electrostaticdischarge (ESD) polyurethane rods that are machined to the desired shapeand size after casting. Alternatively, a 67D polyester-typeThermoplastic Polyurethane (TPU) such as ESTANE (™ of Lubrizol AdvancedMaterials, Inc., Cleveland, Ohio) 58137 TPU. The wheel, in theillustrated embodiment, is substantially cylindrical, though, as can bemore clearly seen in FIG. 2, has an outer periphery that is inclinedwith respect to an axis of symmetry 24 centered within the respectiveaxle 18. Specifically, the radius of the wheel at either a front edge 11or rear edge 13 is less than the radius measured closer to the middle ofthe wheel. This difference in radius can be linear or curved, the latterbeing illustrated in the figures.

The wheel 10 is disposed upon a bearing assembly 16 of conventionaldesign and configuration. The bearing assembly 16 is disposed about anaxle 18 that projects from a drive rail 20. The axle is shown as beingthreaded in the figures, and can be mated with a complimentarilythreaded bore in the drive rail. However, the axle may be mechanicallymated with respect to the drive rail in any conventional manner. Thedrive rail is shown as being L-shaped in FIG. 1, though it can beprovided in a variety of shapes.

Disposed about the wheel outer peripheral surface is a slot 12. As shownin FIG. 2, the slot is continuous about the periphery of the wheel toform a ring-shaped or circular slot into which is provided a ring ofpliant material 14. In a first, illustrated embodiment, the slot and thering of pliant material are rectangular in cross-section, though inother embodiments, different geometries can be utilized. For example, inanother embodiment, the pliant ring may have a circular or ovoidcross-section, while the slot has a complimentary semicircular orsemi-ovoid cross-section. The pliant ring is preferably configured tohave a maximum thickness, measured in the radial direction of the wheel,that is slightly greater than the maximum depth of the slot. Thus, thepliant ring normally extends a distance x beyond the proximal surface ofthe wheel itself. The pliant ring is provided of polyurethane in a firstembodiment, though other soft, compressible, non-friable materials canbe used. Such other materials may include silicone and rubber. In apreferred embodiment, the pliant ring is stretched and forced over thewheel outer periphery and into the slot. The diameter of the pliant ringat rest may be less than the diameter of the slot, such that the pliantring is held in place through friction fit in one embodiment. In otherembodiments, the pliant ring is held in place through an adhesive bondor through mechanical means, including friction fit between the sidewalls of the pliant ring and the side walls of the slot (not shown).

In FIG. 3, a drive belt 22 is shown in cross-section, disposed acrossthe top of the pliant ring 14. This is also depicted in greater detailin FIG. 4. When there is no carrier or other item being transported onor proximate to the respective wheel, the belt lower surface remains incontact at least with the upper or outer surface of the pliant ring 14,whereby the respective wheel may respond immediately and withoutslipping to movement of the belt. Should a wheel have a defect in anouter extent thereof, or if an axle 18 is bent or otherwise notorthogonal to a drive rail, the belt may also at times come into contactwith the outer surface of the wheel itself. However, the pliant ring isintended to ensure that the belt is always in contact with therespective wheel, either directly or indirectly, in order to avoidparticulation resulting from intermittent contact between the belt andwheel.

The choice of materials for the drive belt 22 depends in part upondesired values for durometer and electrical conductivity. Pyrathane83ASD and Stat-Rite S-1107 are typical belt materials. A belt ofPyrathane is somewhat softer and more elastic but simultaneously lesselectrically conductive. A belt of Stat-Rite is harder and more stiff,but simultaneously more electrically conductive. Preferably, theelastomeric belt is stretched onto the wheels and serves to directlytransport overlying work piece carriers through interaction with all ofthe idler and drive wheels.

Once a carrier (not shown) is on the belt 22 above or proximate aparticular wheel 10, the weight of the carrier is sufficient to compressthe pliant ring 14 such that the belt 22 undersurface comes into directcontact with the relatively hard surface of the wheel outer surface, asshown in FIG. 6. The hardness of the wheel ensures that the belt doesnot dip as the weight of the carrier traverses each wheel and insteadprovides a level, smooth transition for the carrier. In addition, theincreased area of contact between the belt lower extent and the wheelperiphery, compared to the area of contact between the belt lower extentand the pliant ring periphery, ensures sufficient frictional force toachieve accurate rotational tracking between the belt and wheel.

In FIG. 5, a perspective view of one embodiment of a drive segment canbe seen. The length of a conveyor segment is determined by a number ofdrive segments it comprises. A drive segment is defined as the length ofa work piece carrier plus some margin of free space. Thus, dependingupon the embodiment, a conveyor segment may be configured to hold one,two, or more drive segments. With this modular approach, the designer ofa conveyor application then constructs the conveyor layout usingstandard, prefabricated modules of length which hold an integral numberof drive segments each. This methodology allows easy conveyor networkdesign and assembly.

In the figure, a linear array of wheels 10 is provided in relation to adrive rail 20. In the illustrated embodiment, each such wheel 10 of thearray is provided with a peripherally disposed pliant ring 14 to improvethe degree of rotational contact between the wheels and an overlying,continuous belt 22. In this illustrated embodiment, each of the wheels10 in the linear array across the conveyor segment are idler wheels. Inother words, each of the wheels of the linear array are unpowered andare rotated through continuous contact with the overlying belt. Notethat in other, more simplified embodiments, the idler wheels arecrowned, as shown in FIGS. 1 and 2, but are not provided with a slot 12or pliant ring 14. Further still, in yet other embodiments, some or allof the idler wheels have a flat outer surface, parallel to the axle 18,upon which a respective belt 22 rolls.

At opposite ends of the linear array, the belt 22 extends slightly lessthan 180 degrees about respective end wheels 10 in substantially theopposite direction towards two lower return idler wheels 26. The beltextends approximately 90 degrees about these return wheels and thenceabout the upper surface of a drive rod 28. Each of the return wheels 26and the drive rod 28 may also be provided with a respective pliant ring14 in an alternative embodiment, while in other embodiments, one or bothdo not have a respective pliant ring.

In this illustrated embodiment, the drive rod 28 is selectively rotatedby a motor 56 (FIG. 8) according to techniques known in the art. Byrotating one end of the drive rod by operation of the motor, cooperatingbelts on opposite sides of the conveyor segment are rotated in unison,thus resulting in linear, even transport of a carrier disposed on anupper surface of the two belts. The drive shaft in one embodiment is acombination of shaft and universal coupling to allow some degree ofmisalignment between the two sides of the conveyor rail. For example,with respect to FIG. 7, the drive shaft 28 is provided with a flatprotrusion on each end, with the protrusion 40 on the proximate end inthe drawing being orthogonal to the protrusion 42 on the opposite,distal end. The flat protrusion on one end of the drive shaft fits intoa slot 52 in the center of a respective drive wheel 50 mounted on onerail frame 20 (not shown in FIG. 8) and to a motor 56 by a spindle 54,as shown in FIG. 8, while the opposite flat protrusion fits into arespective slot in the center of a respective slave wheel on the other,parallel rail frame. The slave wheel is rotatable about a respectivespindle through bearing means known in the art.

Through the use of a common drive shaft, the conveyor belts on bothsides of the conveyor segment are synchronized to run at identicalspeeds, thus avoiding the twisting of work piece carriers on top of thebelts as they travel across the conveyor segment. As shown in FIG. 8,the drive wheel 50 and slaved drive wheel on the opposite end of thedrive shaft have identical cylindrical shapes. Importantly, the radius Rof each drive wheel is identical. This assures that the left and rightbelts are driven at identical speeds, in spite of the normal tendency ofthe belts to each seek its own highest tension by locating themselves onthe highest point of the idler wheels' crowns.

Due to material variations, conveyor load accelerations, frictionalcoefficient differences, belt sizes, and mainly the imperfections inwheel shaft alignments, such that not all wheel axes of rotation are notperfectly parallel with each other, the left and right belts normallywould otherwise run at slightly different speeds. This would beproblematic in clean environments where such speed differentials couldlead to friction and particulation. The cylindrically shaped drivewheels counteract this tendency and equalize belt speeds on the twosides.

In an alternative approach, the conveyor belt is a timing belt, having aflat surface presented upwards towards work piece carriers travelingthereon. The inner surface of the drive belt is provided with mechanicalfeatures that cooperate with complimentary mechanical features on theouter periphery of the idler wheels. Specifically, in a first embodimentof such a timing belt, the inner surface of the belt is provided with alinear and continuous array of projections such as pyramidal orfrusto-pyramidal projections and the idler wheels are provided with alinear array of complimentarily shaped apertures, each configured toreceive a respective belt projection as it passes over the idler wheel.In a second embodiment, the projections, such as pyramidal orfrusto-pyramidal projections, are formed in a linear band about theouter periphery of the idler wheels, while the belt is provided withcomplimentarily shaped and spaced apertures adapted to receive the idlerwheel projections as the belt travels over the idler wheels. In thissecond embodiment, the belt apertures may extend through the belt to thework piece carrier contact surface or, if the belt is of sufficientthickness, may only extend partway through. In any such embodiment,however, the timing belt ensures the idler wheels continuous rotate insync with the overlying belt and particulates are avoided through theavoidance of intermittent belt/wheel contact.

Centering wheels 30 are provided to center the carrier on the belts, inthe illustrated embodiment. One or more intermediate idler wheels 32 mayalso be employed where the placement of the drive rod 28 results in agap between adjacent idler wheels 10 in the linear array. Suchintermediate idler wheels may or may not be provided with pliant rings,as disclosed.

In other embodiments, one of the wheels 10 at either end of the lineararray may be powered, or one of the return wheels 26 may be powered,instead of the drive rod as shown. This, however, would require driveelements such as motors on opposite sides of the conveyor segment.Keeping two such motors perfectly synchronized in terms of start or stoptimes and rotational speed may be a technical challenge.

Alternatively, the drive rod 28 may replace pairs of wheels 10 onopposite sides of the conveyor segment, such as at one end of the lineararray of wheels, or one pair of return wheels 26. The drive rod asdepicted in FIG. 5 would then be replaced by idler wheels on oppositesides of the conveyor segment. Further still, plural drive rods could beemployed, though again this would require accurate synchronization ofdrive elements associated with each such drive rod.

In the illustrated embodiment, a hysteresis clutch is not employed inconjunction with the motor 56 for avoidance of slippage between a workpiece carrier and the belts. In addition, each drive segment is providedwith at least one sensor 60, and preferably at least two sensors, fordetecting the presence of one or more work piece carriers within theconveyor segment. With at least two sensors, one sensor can be providedproximate each end of the respective drive segment such that therespective controller can know whether a work piece carrier occupies thedrive segment. Such sensors are of conventional design and can includethe use of optical, magnetic, passive resonant circuit, weight,mechanical interference, and inductive sensors.

The one or more sensors associated with one conveyor drive segment arepreferably in communication with a local controller 58 associated withthe respective conveyor segment drive motor 56. The controller ispreferably provided with a communications interface and is incommunication with the respective controllers of the at least oneconveyor segments on either side thereof, such as via a communicationsbus of conventional design and configuration. In one embodiment, the busis an industrial Controller Area Network (CAN) bus. Obviously, if theconveyor segment is a port, such as an interface to a process tool, therespective controller would only communicate with the one adjacentconveyor segment controller.

Multiple segment-specific controllers are in communication with arespective higher-level controller. This higher level controller has amap of the conveyor segment for which it is responsible, and isprogrammed with the ability to direct how each carrier within thisconveyor domain are to be routed. This information is used to controlthe response of the individual segment-specific controllers. Dependingupon the complexity and size of the overall conveyor system, multiplelevels of higher-order controllers may be employed.

The controller for each drive segment is thus capable of detecting thepresence of a work piece carrier in an adjacent drive segment and canreact to receipt of a new work piece carrier accordingly, such as bydecelerating that work piece carrier and bringing it to a stop to avoida collision with a downstream carrier. The controller is also capable ofdetecting the movement of a previously stationary work piece carrier inan adjacent drive segment and can respond by accelerating a work piececarrier contained within the respective segment from a stopped conditionor can continue transporting the work piece carrier through that drivesegment to the next.

Acceleration and deceleration profiles are preferably stored in a memory62 associated with the local conveyor segment controller. These profilesmay be standard profiles to be used for changing work piece carrierspeed, or may be maximum values, whereby the controller is programmed tohave flexibility in adjusting work piece carrier speed according to thepresence or absence of carriers within the respective conveyor drivesegment and/or within adjacent conveyor drive segments.

The drive segment, as defined above, is approximately the same length asa work piece carrier, plus a small measure of free space. Thus, for a300 mm wafer carrier found in semiconductor manufacturing environments,a drive segment is 0.5 meter in length. A typical carrier in asemiconductor manufacturing environment has a mass of approximately 8.5kg and can travel at speeds of approximately 1 meter per second. Adeceleration profile must be selected to enable deceleration of thismass to a stop before it enters a downstream, occupied drive segment.This deceleration profile is generally linear in a first embodiment.

However, it is also envisioned in a further embodiment to use anexponential deceleration profile, where the rate of change in speed isslow at the start but greater at the end, near the stopping point. Thistakes advantage of the speed-torque characteristic of stepper motors:generally, motor torque in stepper motors is higher at low speeds.

While deceleration profiles have been discussed in the foregoing,similar profiles can be employed for acceleration to achieve maximumacceleration without slippage. Such controller acceleration anddeceleration profiles enable work piece carriers to travel at highspeed, in very dense flow environments, without the possibility ofcollisions.

While in the foregoing only adjacent drive segments and/or conveyorsegments are described as being in mutual communication, controllers ofa larger range of nearby drive or conveyor segments can be in mutualcommunication to enable faster response to segment occupancy changes andto enable predictive response.

Many changes in the details, materials, and arrangement of parts andsteps, herein described and illustrated, can be made by those skilled inthe art in light of teachings contained hereinabove. Accordingly, itwill be understood that any following claims are not to be limited tothe embodiments disclosed herein and can include practices other thanthose specifically described, and are to be interpreted as broadly asallowed under the law.

What is claimed is:
 1. A wheel for supporting a drive belt in a beltedconveyor system, the wheel comprising: a substantially cylindrical wheelhub having a radially-extending outer periphery, the outer peripheryhaving a front edge and a back edge; a slot formed in the wheel hubouter periphery between the front edge and the back edge; and a ring ofpliant material disposed within the slot, wherein the maximum thicknessof the ring of pliant material, measured in the radial direction, isgreater than the maximum depth of the slot, measured in the radialdirection.
 2. The wheel of 1, wherein the slot has a rectangularcross-section.
 3. The wheel of 1, wherein the ring of pliant materialhas a rectangular cross-section.
 4. The wheel of 1, wherein the wheelhub has a hardness greater than a hardness of the ring of pliantmaterial.
 5. The wheel of 1, where in the ring of pliant material isfriction fit within the slot.
 6. The wheel of 1, where the diameter ofthe outer periphery of the wheel hub increases from the front and backedges towards the center of the periphery.
 7. The wheel of 1, furthercomprising a bearing assembly.
 8. A conveyor segment, comprising: adrive rail; a plurality of idler wheels disposed in a linear array fromthe drive rail; a drive wheel disposed from the drive rail; and acontinuous belt disposed above the plurality of idler wheels and aboutat least a portion of the drive wheel, wherein each of the plurality ofidler wheels comprises a peripherally formed slot and a ring of pliantmaterial disposed within the slot and projecting therefrom, the ring ofpliant material in contact with an underside of the belt, the wheelshaving a material hardness greater than that of the ring of pliantmaterial.